Ok my problem is, that I rented a vehicle for travelling to a watercity. But then I realized that it isn?t able to travel on water.
So I went back to the guy who rents them and wanted to rent a vehicle which is able to travel on water.
But the npc said that I?ve already rented a vehicle and so I cannot rent another one.

To my knowledge, you can't rent a vehicle which travels on water, nor would you have any way to get it to the water in the first place. If you want to cross that terrain, you might want to just travel around it instead, by heading in a different direction than the one you were traveling in.